1. Field of the Invention
The present invention relates to stabilizers for pipe strings in wellbores, and, in particular, a casing stabilizer.
2. Description of the Related Art
In the drilling of wells, such as those for oil and gas, a string of tubular members is threaded together to form a drillstring having a drill bit mounted on the distal end. The drill bit is rotated either from the earth's surface by rotating the drillstring or by a downhole motor.
To enhance well drilling operations, numerous tool have been developed for mounting and use at sub-surface locations in the drillstring. One such tool is a stabilizer. Stabilizers include various forms of centralizers. A centralizer contacts the borehole wall and effectively serves as a radial bearing or lateral support for the drillstring in the borehole. By holding the drillstring against lateral forces or radial movement, the centralizer acts along the unsupported column length of the drillstring to prevent buckling. The centralizer also reduces the bending stresses induced by movement of the drillstring. With the development of casing drilling, where the casing is used as the drill string and remains downhole as the wellborn liner, it is important that the integrity of the casing be maintained.
In conventional drilling, centralizers and other forms of stabilizers are usually formed by a tubular member with a plurality of outwardly extending fixed blades having wall contacting surfaces of hardened material that bear against or contact the sides of the borehole. The outwardly extending blades are usually mounted vertically or in a helical arrangement.
There are generally two major categories of centralizer used with casing strings. The first includes centralizers having blades that are essentially permanently connected. These centralizers are subject to wear, and during refurbishment, often require the addition of hard-facing which can result in a heated effected zone which leads to stress crack propagation. The second includes centralizers that are connected to an outer surface of the casing string. The means for connecting the centralizers to the casing string vary widely. One type of connection means includes the use of threaded connections which are inserted into the casing string at regular intervals by threading to the casing pipe threads in a conventional manner. Centralizers that thread into the casing string are very expensive and are not convenient to use since they must be selected to fit exactly to the connection type being used.
Another means for connecting the centralizers include locking collars to secure the centralizer to the casing or other drillstring. A locking collar uses set screws that engage into the material of the pipe. Through the locking collar, the centralizer is prevented from moving axially and from relative rotation on the pipe. However, a centralizer including a locking collar with set screws is relatively weak and sometimes cannot withstand the harsh drilling environment. In addition, the set screws damage the casing pipe, reducing its strength.
Another means for connecting the centralizers include use of a crimping device whereby portions of a tubular part of the body of the centralizer are crimped at periodic intervals to cause an interference (press) fit. Most types of such centralizers, however, include a tubular body which slides over an outer diameter of the casing string, and thus, is generally slipped over an end of the string. Further, although effective, such crimp on centralizers are generally not available for oil casing greater than 13 ⅜ inches.
Recognized by the inventor is the need for an improved centralizer/stabilizer design that does not require a permanent connection and means for connecting the centralizer/stabilizer that does not require threaded connections or locking collars, that can be used on both large and small casing sizes, and that can be connected to the outer diameter of an existing casing string without a need to access an end of the casing string.